Review of Stone Dust Application in Queensland Underground Coal Mines

Stone dust application prevents coal dust explosions in underground coal mines. It does this by stopping oxidative reactions from taking place. The most commonly used form of stone dust is limestone. Coal dust explosions occur as a result of self-heating or spontaneous combustion. In underground coal mines coal dust explosions are a long-standing thermal dynamic hazard. Different physical and chemical processes contribute to coal dust explosions. The use of stone dust in mine roadways is a primary control against the explosion hazard. It is a critical part of the mines safety and health management system. In Queensland, coal dust explosions are prescribed as regulation. Coal mining operates under a risk-based legislative framework considered under sections of the Queensland Coal Mining Safety and Health Regulation. These are sections 300, 301, 302 and 303 which provide specific requirements to control coal dust explosions. Additionally, Recognised Standard 5 was developed by the Queensland Mines Inspectorate to provide guidance for compliance with the Coal Mining Safety and Health Regulation. In this article we review stone dust functions, properties and specifications focussing on coal dust explosions and compliance in Queensland. The article wraps up with industry best practice for roadway dust in underground coal mines.

Stone dust- functions, properties and specifications
Stone dust application is the use of limestone or dolomite as explosion inhibitors in underground coal mine roadways. It acts as a dilutant for combustible levels of coal dust. Application of stone dust also performs as an adsorbent of available heat. This obstructs oxygen and other gases from participating in coal dust explosions. There are specifications for limestone dust used in stone dust applications. The stone dust should be incombustible. Stone dust used for explosion suppression must be light in colour. Unless wetted by water, it must not cake and must disperse in air. Stone dust must not contain more than 3% by mass of free silica as determined by MDG3006 MRT5. It must not contain any other toxic substance in concentrations detrimental to health. The size range limitations need limestone dust to have a certain level of fineness. On size analysis the stone dust must:

• Not be less than 95% by mass
• Pass through a 250 micrometre sieve
• The dry dust which passes the sieve must be between 60% and 80% by mass passing through the 75 micrometre sieve.

Specifications are compulsory and the stone dust quality control results must comply. In general stone dust application and re-application is as often as necessary with a minimum frequency rate of application not less than once in every four production shifts unless a risk assessment indicates otherwise. The same applies to roads within the face area of the underground coal mine and roads carrying return air.

Coal dust explosions- mechanisms and understanding the hazard
The interaction of coal with oxygen at low temperature gives off heat. Some reaction sequences take in heat. Low temperature oxidation is the main source of heat leading to spontaneous ignition resulting in coal dust explosions. Microbial metabolism, coal-water interaction and oxidation of pyrite contribute to self-heating of coal. Heat given off as moisture bonds with dry coal particles releasing heat of wetting. Th heat generated by coal oxidation must be dissipated. Processes such as conduction, convection and radiation are responsible for dissipating heat. Failure to dissipate heat results in coal dust explosions. Coal stores excessive heat owing to its poor thermal conductivity, which results in net increase in temperature. As a result the reaction rate of coal oxidation increases making it favourable for coal dust explosions. Upon reaching a critical coal temperature value, the thermal runaway occurs and a coal dust explosion leading to a fire ensues. The hazard of self-heating burns valuable coal and poses a catastrophic safety and health hazard to mine workers. Coal dust explosions result in loss of its calorific value and coking property. Environmentally, coal dust explosions liberate large amounts of greenhouse gases some of which are acutely toxic due to hazardous substances such as arsenic, selenium and lead.

Balancing act between compliance in Queensland and stone dust quality
In Queensland, coal dust explosions have happened in underground coal mines with investigations concluding that the problem was lack of adequate and effective stone dusting. This led to failure in containing the coal dust explosions – with devastating results. An audit on efficacy of coal dust explosion prevention systems highlighted challenges. Legislative requirements for sampling, analysis and application of stone dust were misunderstood. This resulted in non-compliance in all those critical aspects. The correct way was checking of stone dust quality as required by Recognised Standard 5. Such actions should include:

• Each batch supplied should have a statement of compliance from the supplier enabling corrective action in the event of non-compliance to the quality rules.
• Quality checks must be done before supply or application.
• Each batch supplied should be checked for quality compliance and a certificate issued.
• An independent test laboratory must verify the quality of stone dust supplied.
• Mandatory monitoring and reviewing the efficacy of stone dust quality.

Where these points are not considered and implemented, risk of ineffective application of stone dust remains if compliance requirements are not met.

Industry best practice for roadway dust in underground coal mines.
Stone dust treatment systems should be adequate and effective to ensure that stone dust quality meets the compliance requirements based on legislation. To prevent coal dust explosions, stone dust must contain non-combustible material. Spontaneous combustion results in health and environmental hazards. Prevention of self-heating of coal is a priority in underground coal mines with longwall tail gate or return roadway the most vulnerable areas. They are some areas of the mine that suffer from limitations to application of stone dust. This happens as a result of deposition of float coal dust and poor accessibility. Suitable methods for application of enough stone dust must be in place to enable stone dust to inhibit coal dust explosions. The mines’ standard operating procedure should cater for:

• Minimising risk of a coal dust explosion
• Suppressing a coal dust explosion
• Limiting its propagation
• Collection and removal of excessive coal dust from roadways
• Stone dusting must fit within an integrated, risk-based site management plan that focuses on efficient production within a safe working environment.

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REFERENCES

• Colinet et al. 2010. Best Practices for Dust Control in Coal Mining. Information Circular 9517. 
• Goertz, B. 2017. Recommendations for the prevention and suppression of coal dust explosions at underground coal mines in the United States. Master’s Thesis. 
• Li, S., Ma, X., and Yang, C. 2018. Prediction of spontaneous combustion in the coal stockpile based on an improved metabolic grey model. Process Safety and Environmental Protection.
• Zhang et al. 2016. A review on numerical solutions to self-heating of coal stockpile: Mechanism, theoretical basis, and variable study. Fuel. 182. 80-109.